That completely depends on the subject and on your expectations. A PhD is not very well-defined in terms of energy you have to invest, nor is it a good indication of intellectual capacity needed to complete it in a reasonable amount of time. PhD programs are just too different to say anything meaningful about these things. Also your supervisor plays an important role, which is usually another unknown variable.
It works really well if there's some synergy between your job and PhD. For example, a friend of mine is working at an R&Dish job while doing a PhD. His thesis is essentially an excuse to dive more deeply into the problems he's currently trying to solve during working hours. Everybody wins!
Barring that, I think it's going to be difficult or impossible. My current (wet) lab would not consider a part-time PhD student. Most of the experiments we do are time-consuming and can't really be done piecemeal like that. Theoretical stuff works a bit better, but I think there is a lot to be said for being "around."
That completely depends on the subject and on your expectations. A PhD is not very well-defined in terms of energy you have to invest, nor is it a good indication of intellectual capacity needed to complete it in a reasonable amount of time. PhD programs are just too different to say anything meaningful about these things. Also your supervisor plays an important role, which is usually another unknown variable.